66 
FISSURES NEAR WIRKSWORTH. 
examined with him in the Dream Cavern and the Fox Holes, and 
that almost all these apertures occur in elevated situations, where not 
a stream or rivulet exists at the present time, to the flood waters of 
which it would be possible to refer their introduction. The absence 
of pebbles and stony fragments in the interior of the cave of Fox 
Holes may be explained by the circumstance of there being no fissure 
communicating from it upwards to the surface, whilst its mouth is 
placed in a nearly vertical cliff* that faces inwards to the valley. 
In the crag called Yeo-ClifF, immediately on the west of Wirks- 
worth, and in other natural cliffs that occur on the north side of the 
same town, are lofty precipices formed by the edges of the metalli¬ 
ferous limestone strata. These are seen to be intersected from top 
to bottom by numerous and nearly vertical fissures or veins, many of 
which are still filled with various kinds of spar and ore; others par¬ 
tially empty, in consequence of the removal of these substances by 
the operations of mining; and others again wholly or partially filled 
with diluvial detritus. These metalliferous limestone strata compose 
the upper table-lands of much of this district, and have been for cen¬ 
turies ransacked in pursuit of lead. They are in all parts intersected 
by fissures similar to those exposed in the cliffs near Wirksworth, 
and which like them are either filled with spar, and form the most 
prolific deposits of metallic ore, or are choked up with mud and rocky 
fragments drifted in by diluvial agency, or (which is but rarely the 
case) remain still open, and have apparently been never filled with 
any thing since the fracture of the rock took place to which they owe 
their origin. In these last we find no other extraneous substances 
